Flow control valve with single spool second stage

ABSTRACT

A two stage valve system is described in which the rate of flow of fluid to the load depends solely on the input signal to the first stage. Both stages are spool type valves. The second stage is formed to provide a passageway to allow fluid to flow from the first stage to the load, which passageway is opened simultaneously with the opening of the usual passageway from the second stage supply to the load. Additionally, the second stage supply is connected through restricted passageways to the second stage end spaces.

United States Patent [191 Byers FLOW CONTROL VALVE WITH SINGLE SPOOLSECOND STAGE [75] Inventor:

[73] Assignee: Sanders Associates, Inc., Nashua,

[22] Filed: Feb. 24, 1972 [211 App]. No.: 229,044

James Otto Byers, Manchester, NH.

[52] US. Cl. 13 7/625.63 [51] Int. Cl. Fl6k 11/07 [58] Field of Search137/82-86, 625.6-625.69

[56] References Cited UNITED STATES PATENTS 3,561,488 2/1971 Byersl37/625.63 X

[451 July 3,1973

Primary ExaminerHenry T. Klinksiek Attorney-Louis Etlinger [57] ABSTRACTA two stage valve system is described in which the rate of flow of fluidto the load depends solely on the input signal to the first stage. Bothstages are spool type valves. The second stage is formed to provide apassageway to allow fluid to flow from the first stage to the load,which passageway is opened simultaneously with the opening of the usualpassageway from the second stage supply to the load. Additionally, thesecond stage supply is connected through restricted passageways to thesecond stage end spaces.

19 Claims, 8 Drawing Figures SOURCE Z3 FORC E MOTOR PATENIEDJULB 1915FIG. 7

SHEET 2 0F 2 FIG. 6

P2 Pl qal FIG. 8

FLOW CONTROLVALVE WITH SINGLE SPOOL SECOND STAGE FIELD OF THE INVENTIONexerted by the load.

BACKGROUND OF THE INVENTION There are many applications of valves inwhich it is desirable that the rate of flow of fluid to the load beindependent of the load pressure. One approach to the problem, typifiedby the Hayner U.S. Pat. No. 3,126,031, involves measuring the actualrate of flow to the load and feeding back a signal indicative thereof tothe first stage.

Another approach to the problem is that described in U.S. Pat. No.3,561,488, granted Feb. 9, 1971 to the present applicant and entitledFluid Flow Control Valve. Briefly stated, this patent describes a twostage valve in which the flow of fluid to the load includes not only theusual flow from the second stage supply but also includes flow from thefirst stage. As explained in the patent, the rate of flow to the loadfrom the first stage depends solely on the input signal and the rate offlow from the second stage supply is proportional to the rate of flowfrom the first stage. As a result, the total rate of flow to the loaddepends solely on the input signal.

One embodiment (that of FIG. 1) of the invention described and claimedin the above mentioned U.S. Pat. No. 3,561,488 includes a force motorfor controlling a spool type first stage which in turn controls thesecond stage. The second stage itself includes two spools which areactive alternatively, depending upon the sense of the input signal.While this arrangement is entirely satisfactory in operation, the twospool construction is complex, bulky, and requires manyinterconnections. When an attempt is made to combine the function of thetwo spools in a single spool, a number of problems are encountered, notthe least of which is that of the conflicting pressure requirements atthe ends of the spool. In the two spool construction, one end of eachspool is subjected to the pressure of the second stage supply while thetwo remaining ends are subjected to the pressure of the first and secondcontrol lines from the first stage. Accordingly, when a single spooldesign is attempted, it would appear that there are three separatepressures required but only two spool ends to which to apply them.Resolution of these conflicting requirements has hitherto thwarted at-.tempts to substitute one spool for two.

It is an object of the present invention to provide a two stage valvesystem employing a spool type first stage in which the rate of flow offluid to the load depends solely on the input signal but whichnevertheless requires but a single spool in the second stage.

SUMMARY OF THE INVENTION ports and lands to selectively interconnectfirst and second load lines to supply and return lines. The second stagespool is also formed with first and second passageways connectingopposite end spaces to additional passageways which, upon displacementof the spool, are opened to the load simultaneously with the opening ofthe usual passageway from the supply to the load. These passageways aresimilar to those described in the aforementioned US. Pat. No. 3,561,488.Additionally, as one end space is subjected to the flow of fluid fromthe first stage, the other end space is automatically biased by thepressure of the second stage supply. Such biasing may be obtained byvalving or more simply by restrictors such as two orifices formed in thesecond stage spool so as to establish restricted communication betweeneach of the first and second passageways and the interior of the valvebody in the region of the supply port.

DESCRIPTION OF PREFERRED EMBODIMENT For a clearer understanding of theinvention, reference may be made to the following detailed descriptionand the accompanying drawing, in which:

FIG. 1 is a schematic cross section view of a valve system incorporatingthe invention;

FIG. 2 is a cross section view taken on the line 2-2 of FIG. 1.

FIG. 3 is a fragmentary plan view, with the valve body removed, of thatportion of the apparatus which is shown in FIG. 2;

FIG. 4 is a schematic cross section view showing the inventionincorporated into a different valve system;

FIGS. 5, 6, 7 and 8 are schematic diagrams illustrating variousalternative forms for the first stage.

Referring first to FIG. 1, there is shown a two stage valve systemincluding a three way spool type first stage valve, indicated generallyby the reference character 11, and a four way spool type second stagevalve, indicated generally by the reference character 12. The firststage 11 includes a valve body 13 formed to define a hollow cylinder 14.A valve spool is located within the cylinder and is referred togenerally by the designation 15. Particular portions of the spool aredenoted by the reference character 15 plus a letter. More particularly,the spool 15 includes a central land 15a and opposite end lands 15band.l5c all engaging the inside surface of the hollow cylinder 14 andjoined by reduced diameter portions 15d and 15e. The cylinder 14 islonger than the spool 15 thereby defining end spaces 16 and 17 whichcontain centering springs 18 and 19. Both end spaces are connected tothe return line, or reservoir, designated R. The valve body 13 is alsoformed to define a central groove, or port, 21 which is connected by aconduit 22 to a source 23 of fluid having a pressure P1. In the neutral,or reference, position of the spool 15 shown in FIG. 1, the land 15aoccludes the port 21. Two control conduits 24 and 25 communicate withthe interior of the cylinder on opposite sides of the port 21 in theregion of the reduced diameter portions and l5e respectively. Aforce'motor 26 is operatively connected to the spool 15 by means of afurther reduced diameter portion 15f.

The second stage 12 includes a valve body 31 formed to define a hollowcylinder 32and two end spaces 33 and 33'. A spool referred to generallyby the reference character 35, is disposed in the cylinder 32 andineludes lands 35a, 35b, 35c, and 35d engaging the interior surface ofthe cylinder 32 and interconnected by reduced diameter portions 35e, 35fand 353. The end space 33 communicates with the control conduit 24 andcontains a compression spring 36 acting between a flanged washer 37hearing against the end of the cylinder 32 and another flanged washer38. When the spool 35 is in its neutral position, as shown, the innerportion of the washer 38 engages the end of the land 35a while the outerportion, which is of greater diameter than the land 35a, engages ashoulder portion 39 of the valve body 31. The end space 33' communicateswith the control conduit 25 and Contains similar parts, namely, a spring36' and two flanged washers 37 and 38' similarly arranged. The springs36 and 36' are each pre-loaded and provide a definite neutral positionfor the spool 35.

The valve body 31 is formed to define grooves, or ports 41 and 42 whichcommunicate, by means of conduits 43 and 44, with a useful load 45. Inthe neutral position of the spool 35, as shown, the lands 35b and 350occlude the ports 41 and 42 respectively. The interior of the cylinder32 in the regions of the reduced diameter portions 35c and 35g isconnected to the return line R. A source 46 of fluid at a pressure P2,less than pressure P1, is connected to the interior of the cylinder 32in the region between the load ports 41 and 42. The sources 23 and 46may be independent sources, or source 46 may be derived from source 23through a pressure reducing valve, or source 23 may be derived fromsource 46 by means of a pressure booster. What is important is that PIbe greater than P2.

The spool 35 is formed with a longitudinal passageway 51 which extendsfrom the end space 33 to that face 52 of the land 35b which is subjectedto the pressure P2. The spool 35 also includes a protruding portion, orboss, 53 which abuts and extends longitudinally outward a short distancefrom the face 51 and which extends radially from the reduced diameterportion 35f to the interior surface of the cylinder32. As best shown inFIGS. 2 and 3, the boss 53 extends only a short distance in thecircumferential direction and its outer surface is convex andcomplements the interior concave surface of the cylinder 32. The boss 53and the portion 35f of the spool 35 include a slot formed to define agenerally rectangular radial passageway 54 one wall of which is thesubstantially planar face 52 of the land 35b. The radial passageway 54is open at the outer convex surface of the boss 53 and extends to andcommunicates with the longitudinal passageway 51. The arrangement of thelongitudinal and radial passageways as so far described is similar tothat described in the aforementioned U.S. Pat. No. 3,561,488 except thatthe radial passageway in the present case extends along one radius onlyinstead of extending along a diameter, as it does in the patent.Additionally, however, the spool 35, and more particularly the portion35f, is formed to define a restricted passageway, preferably in the formof an orifice 55, which connects the longitudinal passageway 51 withthat portion of the interior of the cylinder 32 which communicates withthe source 36 at the pressure P2.

A longitudinal passageway 51 similar to the passageway 51 extends fromthe end space 33 to that face of the land 35c which is subjected to thepressure P2,. The passageway 51' communicates with a radial passageway54', similar to the radial passageway 54 and formed in a similar manner.An orifice 55' interconnects the longitudinal passageway 51 with thatportion of the cylinder 32 which communicates with the source 36 at thepressure P2.

OPERATION In the absence of an input signal to the force motor 26 allthe parts will be in their respective neutral, or reference, positionsas shown in the drawing. The pressure P2 of the source 46 is transmittedthrough the orifices 55, 55' and the longitudinal passageways 51 and 51'to the end spaces 33 and 33'. The radial passageways 54 and 54 areclosed by the interior surface of the cylinder 32.

Let it now be assumed that an input signal is applied to the force motor26 and that in response thereto the motor 26 displaces the spool 15 tothe left. Such displacement opens a passageway across the land 15a fromthe port 21 to the interior of the cylinder 14 thereby allowing fluid toflow from the source 23 through the conduit 22, the port 21, and thecylinder 14 to the control conduit 25. Since the pressure P1 is greaterthan the pressure P2, fluid flows from the conduit 25 to the end space33', through the longitudinal passageway 51 and the orifice 55 to thesource 46. Such flow of fluid causes a pressure drop to appear acrossthe land 15a and another pressure drop to appear across the orifice 55.The pressure in the end space 33' tends to rise but the spool 15 is atfirst held stationary by the action of the spring 36. Fluid continues toflow, increasing the pressure drop across the orifice 55 and increasingthe pressure in the end space 33'. When this pressure is sufficient toovercome the preloading of the spring 36, the piston 15 is displaced tothe left, thereby partially opening the radial passageway 54 andallowing fluid to flow therethrough to the load port 41. The pressuredrop across the orifice 55 will not increase further but will remainconstant at the value necessary to overcome the spring 36. As the spool35 is displaced further to the left, the increasing flow through thepassageway 54 causes the pressure drop across the land 15a to increaseuntil the pressure in the end space 33' is equal to P2 plus thatnecessary to overcome the spring. Stated mathematically, the pressure inthe end space 33' becomes equal to P2 F/A where F is the force exertedby the spring 36 and A is the effective area of the end of the spool 35on which the fluid is acting. When this pressure is reached, the spool35 stops moving and remains in its then attained displaced position aslong as these pressure conditions remain unchanged.

The operation can be explained by noting various conditions:

1. For every value of input signal to the force motor 26 there is acorresponding displacement of the spool 15.

2. During operation, the pressure drop AP across the land 15a is alwaysstabilized at the same value, namely, P (P FIA), regardless of theextent of spool displacement.

3. For every displacement of the spool 15 there is corresponding rate offlow of fluid through the first stage. Let us designate this rate offlow as 0,.

4. For every rate of flow through the first stage there is acorresponding rate of flow through the passageway 54 to the load. Thisrate is equal to Q, Q, where Q, is the rate of flow through the orifice55. (This flow is small and occurs only because the spring 36 requiresthe pressure in the end space 33' and in the passageway 51 to beslightly greater than P 5. Therefore, for every value of input signal tothe force motor 26 there is a corresponding rate of flow through thepassageway 54 to the load.

6. Simultaneously with the opening of the small passageway 54 to theload port 41 by the displacement of the spool 35, the larger passagewayaround the remainder of the land 35b is also opened, allowing fluid toflow from the source 46 to the load. The cross section areas of thesetwo passageways are measured by the lengths of the respective arcsacross which fluid flows, and since these lengths have a fixed ratio, ascan be seen from FIG. 2, the areas are directly proportional. Thepressure drops across these passageways also correspond and are nearlythe same. The pressure on one side of both is that of the load. Thepressure on the other side of the larger is P2 while that on thecorresponding side of the smaller is P2+F/A. Therefore, for each rate offlow from the first stage through the passageway 54 to the load, thereis an exactly corresponding rate of flow from the source 46 to the load.These two rates of flow are approximately proportional. The relationshipdeparts from linearity only because the pressure in the longitudinalpassageway 51 is greater than P If the springs 36 and 36' were absent,the relavtionship would be exactly linear. But even with the springspresent, the nonlinearity is small because, in comparison with the fluidpressures normally used, the force of the springs is very small,ordinarily being only sufficient to provide a small but definite deadspace in the absence of an input signal so that a small threshold valueof input signal must be exceeded in order to actuate the valve.

Any change in the input signal causes a corresponding change in the rateof flow of fluid to the load; when the input signal is reduced to zero,the pressure in both end spaces 33 and 33' returns to P and the springs36 and 36' return the spool 35 to its neutral position. When a signal ofopposite sense is applied, a comparable but opposite sequence of eventstakes place, with the spools and 35 both being displaced to the rightand with fluid flowing through the passageway 54' and across the land35c to the load port 42.

Referring now to FIG. 4, the invention is shown as applied to slightlydifferent valve. The valve body 31 is the same as in FIG. 1 and isformed with the same hollow cylinder 32, end spaces 33 and 33' and loadports 41 and 42. The same centering springs 36, 36' bearing againstwashers 37, 37' and 38, 38 are included. Control conduits 24 and areconnected to the end spaces 33 and 33' as before. The load 45 isconnected by conduits 43 and 44 to ports 41 and 42 as before. However,the spool, referred to generally by the reference character 61, isdifferent. It includes lands 61a, 61b, 61c and 61d each engaging theinner surface of the cylinder 32 and interconnected-by reduced diameterportions 61e, filfand 61g.

Pressure and return connections are reversed from those shown in FIG. 1.The return line R is connected to the interior of the cylinder 32 nearthe center in the region between the lands 61b and 610. The source offluid at pressure P2 is connected to the interior of the cylinder 32 intwo places, namely, in the region between the lands 61a and 61b and inthe region between the lands 61c and 61d.

The spool 61 is formed with a longitudinal passageway 62, similar to thepassageway 51 of FIG. 1, which extends from the end space 33' to aradial passageway 63, (similar to the passageway 54 of FIG. 1) and whichin turn extends along that face of the land 610 which is subjected tothe pressure P2. A restricted passageway, such as an orifice 64, extendsfrom the passageway 62 to the interior of the cylinder 32 in the regionthat communicates with the pressure P2. The opposite end of the spool 61is formed with complementary passageways 61' and 63, similar to thepassageways 51' and 54' of FIG. 1. Since the two longitudinalpassageways are not required to extend side by side, they may be formedon the axis of the spool 61, as shown, but this is not essential. Theradial passageways 63 and 63' may be formed and defined in a mannersimilar to that described in connection with FIG. 1, and, if desired,each may be duplicated so as to have dual passageways at diametricallyopposite locations, similar to the arrangement of US. Pat. No.3,561,488. The operation of the device of FIG. 4 is substantially thesame as thatof FIG. 1.

As shown in FIG. 5, the invention may also be used with a four way spoolvalve as the first stage. In this schematic showing, the four way valve71 is actuated by a force motor 72. In the neutral position the pressuresource P1 is disconnected, that is, blocked, while the reservoir 73 isconnected to both of the output lines 74 and 75. These lines in turn areconnected through check valves 76 and 77 respectively to the controllines 24 and 25. The force motor 72 variably connects the pressure P1and the reservoir to the lines 74 and in a sense determined by the inputsignal to the force motor 72. The check valves prevent back flow whenthe valve is in neutral. The second stage may be that shown in eitherFIG. 1 or-FIG. 4 and operation is substantially the same.

FIG. 6 shows a slightly different arrangement in which a four way valve81 is arranged to connect the first stage pressure P1 and the lower,second stage pressure P2 to the control lines 24 and 25 in a sensedetermined by the sense of the input signal. The second stage may bethat of either FIG. 1 or FIG. 4 and no check valves are necessary.Additionally, the orifices 51, 51' and 64, 64' could be omitted,although operation is satisfactory either with or without them.Operation is substantially the same as that previously described.

FIG. 7 shows an arrangement in which a fixed predetermined flow rate ineither direction may be obtained repeatedly. In this arrangement, thefour way valve 71 of FIG. 5 is used but the force motor 72 is replacedby two solenoids 91 and 92 each of which, when energized, fully opensthe valve in one direction or the other. The predetermined rates of floware selected by adjusting two variable restrictors 93 and 94 which areinserted between the check valves 76 and 77 and the control lines 24 and25.

FIG. 8 shows an arrangement similar to that of FIG. 7. The valve 81 ofFIG. 6 is actuated by the two solenoids 91 and 92. The output lines 74and 75 are connected directly to the variable restrictors 93 and 94. Thecheck valves are not necessary because all flow is blocked when thevalve 81 is in its neutral position.

From the foregoing it will be apparent that applicant has provided anovel single spool second stage flow control valve. As one end of thespool is subjected to the pressure of the first stage supply, asmodified by flow through the first stage, the other end is automaticallybiased by the pressure of the second stage supply. Such biasing may beobtained by various arrangements of valves and/or restrictors, preferredexamples of which have been illustrated. Other arrangements arepossible. For example, the restrictors may be connected anywhere betweenthe second stage supply and the end spaces and need not be in the formof orifices in the spool, as shown, although at present this is thepreferred arrangement. Many other modifications will occur to thoseskilled in the art. it is therefore desired that the protection affordedby Letters Patent be limited only by the true scope of the appendedclaims.

What is claimed is:

l. A valve system for controlling the flow of fluid to a load whichsystem includes two stages and in which both the first and second stagesare spool valves and in which the second stage valve is constructed todefine and open a passageway from the first stage to the loadsimultaneously with the opening of a passageway from the second stagesupply to the load, characterized in that the valve includes means,operative upon subjection of one end of the second stage spool to thecontrol pressure from said first stage, for automatically subjecting theother end of said spool to the pressure of the second stage supply.

2. A valve system in accordance with' claim 1 in which said meansincludes a restricted passageway interconnecting said second stagesupply with said other end.

3. A valve system in accordance with claim 1 in which said meansincludes first and second restricted passageways each interconnectingsaid second stage supply with one of said ends.

4. A valve system in accordance with claim 1 in which said meansincludes first and second orifices formed in said spool, eachinterconnecting one of said ends with said second stage supply.

5. A valve system in accordance with claim 1 in which said first stagevalve is a four way valve and which includes first and second checkvalves in the control lines between said first and second stagesarranged to prevent the flow of fluid from said second stage to saidfirst stage.

6. A valve system in accordance with claim 1 in which said first stagevalve is a four way valve connected to selectively interconnect firstand second sources of fluid under pressure to opposite ends of saidsecond stage spool in a sense determined by the sense of an inputsignal.

7. A valve system in accordance with claim 1 which includes first andsecond sources of fluid under pressure for said first and second stagesrespectively, said second source being at a pressure less than that ofsaid first source, and in which said means includes said first stagevalve which is a four way valve connected to selectively interconnectsaid first and second sources to opposite ends of said second stagespool in a sense determined by the sense of an input signal.

8. A valve system for controlling the flow of fluid to a load, whichsystem includes first and second spool valve stages and first and secondsources of fluid under pressure for said first and second stagesrespectively, said second source being at a pressure less than that ofsaid first source, and which includes meansfor subjecting, selectively,either end of said second stage spool to a control pressure from saidfirst stage, and in which said second stage valve is constructed todefine and open a passageway from that end of said second stage spool sosubjected to control pressure to said load simultaneously with theopening of a passageway from the second stage supply to the load,characterized in that the valve system includes means, operative uponsuch subjection of control pressure to one end of said spool, forautomatically subjecting the other end of said spool to the pressure ofsaid second stage supply.

9. A valve system in accordance with claim 8 in which said last namedmeans includes a restricted passageway interconnecting said secondsource with said other end of said second stage spool.

10. A valve system in accordance with claim 8 in which said last namedmeans includes first and second restricted passageways eachinterconnecting said second source with one of said ends of said secondstage spool.

11. A valve system in accordance with claim 10 in which said restrictedpassageways each include an orifice formed in said spool.

12. A valve system in accordance with claim 8 in which said first stageincludes a four way valve and which includes first and second controllines connecting said four way valve to said ends of said second stagespool and which further includes first and second check valvesinterposed in said first and second control lines respectively forpreventing the flow of fluid from said second stage to said first stage.

13. A valve system in accordance with claim 8 in which said first stageincludes a four way valve installed to operatively connect said firstand second sources selectively to opposite ends of said second stagespool in a sense determined by an input signal.

14. A valve system for controlling the flow of fluid to a load,comprising, a four way spool valve for use as a second stage valve in asystem employing a spool valve as the first stage, characterized in thatsaid four way valve includes longitudinal passageways in the spoolinterconnecting the end spaces with passageways which are opened to theload conduits simultaneously with the opening of passageways to saidload conduits from the second stage fluid source, and in that there areprovided first and second means each including a restricted passageway,each for interconnecting said sec ond stage supply with one of said endspaces.

15. A valve system in accordance with claim 14 in which said restrictedpassageways are formed transversely in said spool so as to interconnectsaid longitudinal passageways with a space communicating with saidsource.

16. A valve system for controlling the flow of fluid to a load,comprising, a first stage including a first spool moveable in a firsthollow cylinder in response to an input signal for variably opening apassageway from a first source of fluid under pressure to either a firstor a second control conduit, according to the sense of an input signal,a second stage comprising a four way valve including a housing formedwith a second hollow cylinder and having a spool moveable therein, saidhousing being formed with ports for connection to first and second loadconduits, to a return conduit and to a second source of fluid underpressure less than that of said first source, said spool including firstand second lands which occlude said first and second load portsrespectively when said spool is in its neutral position each of saidlands including two faces, one subjected to the pressure of said secondsource and the other subjected to the pressure of said return conduit,said lands being arranged to connect said load conduits to said secondsource and to said return conduit in a sense determined by the directionof displacement of said spool, said second cylinder and housing beingformed to define first and second end spaces at opposite ends thereof,and, means for connecting said first and second control conduits to saidfirst and second end spaces respectively, characterized in that saidsecond spool is formed with first and second longitudinal passagewayscommunicating with said first and second end spaces respectively, andalso communicating with first and second radial passageways,respectively, each comprising a slot in a boss abutting that face ofthat land which is subjected to the pressure of said second source andwhich faces that end space with which its associated passagewaycommunicates, each of said radial passageways extending to and beingopen adjacent to the outer cylindrical surface of its associated land,and in that there are provided first and second means each including arestricted passageway for interconnecting said second source with saidfirst and second end spaces respec- 10 tively.

17. A valve system in accordance with claim 16 in which said restrictedpassageways include first and sec-' ond orifices formed in said secondpool in such locations as to interconnect said first and secondlongitudinal passageways respectively with a region of space within saidsecond cylinder which is subject to the pressure of said second source.

18. A valve system in accordance with claim 16 in which said first stageincludes a three way valve.

19. A valve system in accordance with claim 16 in which said first stageincludes a four way valve connected to open substantially simultaneouslyand variably either a passageway from said first source to said firstcontrol conduit and a passageway from said return conduit to said secondcontrol conduit or a passageway from said first source to said secondcontrol conduit and a passageway from said return conduit to said firstcontrol conduit and which further includes first and second check valvesinterposed in said first and second control conduits respectively andconnected to prevent the flow of fluid from said second stage to saidfirst stage.

1. A valve system for controlling the flow of fluid to a load whichsystem includes two stages and in which both the first and second stagesare spool valves and in which the second stage valve is constructed todefine and open a passageway from the first stage to the loadsimultaneously with the opening of a passageway from the second stagesupply to the load, characterized in that the valve includes means,operative upon subjection of one end of the second stage spool to thecontrol pressure from said first stage, for automatically subjecting theother end of said spool to the pressure of the second stage supply.
 2. Avalve system in accordance with claim 1 in which said means includes arestricted passageway interconnecting said second stage supply with saidother end.
 3. A valve system in accordance with claim 1 in which saidmeans includes first and second restricted passageways eachinterconnecting said second stage supply with one of said ends.
 4. Avalve system in accordance with claim 1 in which said means includesfirst and second orifices formed in said spool, each interconnecting oneof said ends with said second stage supply.
 5. A valve system inaccordance with claim 1 in which said first stagE valve is a four wayvalve and which includes first and second check valves in the controllines between said first and second stages arranged to prevent the flowof fluid from said second stage to said first stage.
 6. A valve systemin accordance with claim 1 in which said first stage valve is a four wayvalve connected to selectively interconnect first and second sources offluid under pressure to opposite ends of said second stage spool in asense determined by the sense of an input signal.
 7. A valve system inaccordance with claim 1 which includes first and second sources of fluidunder pressure for said first and second stages respectively, saidsecond source being at a pressure less than that of said first source,and in which said means includes said first stage valve which is a fourway valve connected to selectively interconnect said first and secondsources to opposite ends of said second stage spool in a sensedetermined by the sense of an input signal.
 8. A valve system forcontrolling the flow of fluid to a load, which system includes first andsecond spool valve stages and first and second sources of fluid underpressure for said first and second stages respectively, said secondsource being at a pressure less than that of said first source, andwhich includes means for subjecting, selectively, either end of saidsecond stage spool to a control pressure from said first stage, and inwhich said second stage valve is constructed to define and open apassageway from that end of said second stage spool so subjected tocontrol pressure to said load simultaneously with the opening of apassageway from the second stage supply to the load, characterized inthat the valve system includes means, operative upon such subjection ofcontrol pressure to one end of said spool, for automatically subjectingthe other end of said spool to the pressure of said second stage supply.9. A valve system in accordance with claim 8 in which said last namedmeans includes a restricted passageway interconnecting said secondsource with said other end of said second stage spool.
 10. A valvesystem in accordance with claim 8 in which said last named meansincludes first and second restricted passageways each interconnectingsaid second source with one of said ends of said second stage spool. 11.A valve system in accordance with claim 10 in which said restrictedpassageways each include an orifice formed in said spool.
 12. A valvesystem in accordance with claim 8 in which said first stage includes afour way valve and which includes first and second control linesconnecting said four way valve to said ends of said second stage spooland which further includes first and second check valves interposed insaid first and second control lines respectively for preventing the flowof fluid from said second stage to said first stage.
 13. A valve systemin accordance with claim 8 in which said first stage includes a four wayvalve installed to operatively connect said first and second sourcesselectively to opposite ends of said second stage spool in a sensedetermined by an input signal.
 14. A valve system for controlling theflow of fluid to a load, comprising, a four way spool valve for use as asecond stage valve in a system employing a spool valve as the firststage, characterized in that said four way valve includes longitudinalpassageways in the spool interconnecting the end spaces with passagewayswhich are opened to the load conduits simultaneously with the opening ofpassageways to said load conduits from the second stage fluid source,and in that there are provided first and second means each including arestricted passageway, each for interconnecting said second stage supplywith one of said end spaces.
 15. A valve system in accordance with claim14 in which said restricted passageways are formed transversely in saidspool so as to interconnect said longitudinal passageways with a spacecommunicating with said source.
 16. A valve system for controlling theflow of fluid to a load, comprising, a first stage including a firstspool moveable in a first hollow cylinder in response to an input signalfor variably opening a passageway from a first source of fluid underpressure to either a first or a second control conduit, according to thesense of an input signal, a second stage comprising a four way valveincluding a housing formed with a second hollow cylinder and having aspool moveable therein, said housing being formed with ports forconnection to first and second load conduits, to a return conduit and toa second source of fluid under pressure less than that of said firstsource, said spool including first and second lands which occlude saidfirst and second load ports respectively when said spool is in itsneutral position each of said lands including two faces, one subjectedto the pressure of said second source and the other subjected to thepressure of said return conduit, said lands being arranged to connectsaid load conduits to said second source and to said return conduit in asense determined by the direction of displacement of said spool, saidsecond cylinder and housing being formed to define first and second endspaces at opposite ends thereof, and, means for connecting said firstand second control conduits to said first and second end spacesrespectively, characterized in that said second spool is formed withfirst and second longitudinal passageways communicating with said firstand second end spaces respectively, and also communicating with firstand second radial passageways, respectively, each comprising a slot in aboss abutting that face of that land which is subjected to the pressureof said second source and which faces that end space with which itsassociated passageway communicates, each of said radial passagewaysextending to and being open adjacent to the outer cylindrical surface ofits associated land, and in that there are provided first and secondmeans each including a restricted passageway for interconnecting saidsecond source with said first and second end spaces respectively.
 17. Avalve system in accordance with claim 16 in which said restrictedpassageways include first and second orifices formed in said second poolin such locations as to interconnect said first and second longitudinalpassageways respectively with a region of space within said secondcylinder which is subject to the pressure of said second source.
 18. Avalve system in accordance with claim 16 in which said first stageincludes a three way valve.
 19. A valve system in accordance with claim16 in which said first stage includes a four way valve connected to opensubstantially simultaneously and variably either a passageway from saidfirst source to said first control conduit and a passageway from saidreturn conduit to said second control conduit or a passageway from saidfirst source to said second control conduit and a passageway from saidreturn conduit to said first control conduit and which further includesfirst and second check valves interposed in said first and secondcontrol conduits respectively and connected to prevent the flow of fluidfrom said second stage to said first stage.